Waxes are incorporated into electrophotographic toners to aid release from the fuser roller surfaces, without the need to use silicone oil. The domain sizes of these materials affect the many properties of the toner, including the powder flow, tribocharging, release and glossing. The domain sizes can be effectively controlled by a separate step of milling the waxes to form a dispersion. For this purpose, it is beneficial to mill the wax in a volatile organic solvent that can be removed after the wax is added to the toner. While the wax domain size can be reduced in milling without an aid, it is usually desirable to have milling aid that can act as a colloidal stabilizer for the wax particles. The stabilizer prevents the agglomeration of the wax particles thereby increasing the efficiency of milling and also reduces the viscosity of the dispersion, thereby allowing a higher wax yield and consequently lower milling costs.
Several patents (U.S. Pat. No. 5,756,244) disclose a method of incorporating wax into to the toner at the compounding step, where all the ingredients of the toner and the wax are added to the equipment such as an extruder, two roll mill, kneader etc. In the molten state of the processing conditions, the viscosities of the wax and the toner binder are typically very different. Under such diverse viscosities, it is not possible to achieve small particles of the wax. The size of the wax domain in the extruded matrix may be equal or smaller than final desired toner size. As a result, when the extruded matrix is pulverized the resulting particles may contain free wax particles or non-homogeneous distribution of wax in the particles, with a high content of surface wax. The surface wax and free wax can adversely affect the performance of the toner in the machine.
It has been found that incorporating fine particles of wax as a dispersion into toners made by a chemical manufacturing process results in toners that have little free wax and low surface wax. There are different methods of making toner by the chemical method. In U.S. 20040161687 A1, the wax particles obtained from the vendor are small in size and are added to a solvent phase containing the rest of the toner materials. This organic phase is dispersed in an aqueous phase containing a water soluble surfactant and viscosity modifier to aid in the dispersion. However, the choice of wax which are manufacturable at the desired small size is limited.
Another method of controlling a domain size of the wax is to mill the wax within a solvent medium. U.S. Pat. Nos. 6,682,866; 6,800,412; and 6,824,945; and U.S. Patent Application No. 2004/0115551 disclose combining a polyester binder, pigment, charge control agent and wax and milling it in the presence of zirconia beads. The organic phase is then dispersed, with controlled shearing, in an aqueous phase containing hydroxylapatite and sodium dodecyl benzene sulfonate as dispersants. The solvent is then removed by evaporation. While this method is useful for creating small domain sizes of the waxes, it is not cost effective to process the entire toner/wax/solvent mixture to reduce the domain size. Secondly, polyesters binder do not adsorb well on to waxes, particularly polyolefin and polyester waxes which results in high degree of flocculation and difficulty in milling.
Other methods of incorporating wax via chemical toner processes are by emulsion aggregation technology. In these cases aqueous dispersions of wax, latex, pigment and charge control agent are mixed in a reactor and aggregated to form toner sized particles. Aqueous dispersions of wax can be prepared by several methods. U.S. Pat. Nos. 6,849,371 and 6,210,853 disclose the preparation of wax dispersions by using a sulfonated polyester as a dispersant, which is also the toner binder, raising the aqueous dispersion temperature to above the melting point of the wax, using a high pressure reactor and then emulsifying the wax. U.S. Pat. No. 6,808,851 discloses a similar preparation method with an anionic surfactant as the stabilizer. U.S. Patent Application No. 20040044108 A1 describes the details of preparing the wax dispersions. While aqueous dispersions of waxes are environmentally more benign than solvent based dispersions, the cost of carrying out high pressure homogenizations is relatively high. Furthermore it is substantially more difficult to carry out the emulsion aggregation process and incorporate the wax, than by using a solvent to dissolve and disperse the toner components.
It is required that dispersions of waxes in solvents be prepared, for incorporating into toners prepared by certain chemical processes. The dispersions are prepared by breaking down the wax particles into sub micron particles using milling media. The cost of preparing these dispersions is proportional to time required to reach the final particle size. Therefore it is beneficial to have a high solid loading in these dispersions, for the reasons of reducing the cost. The choice of dispersant determines the rate of milling. Typically, dispersants that enable fast milling are good stabilizers. That is, they adsorb strongly to the wax particles, and provide a good barrier to particle flocculation. While flocculated dispersions can be milled, they are typically higher in viscosity and hard to handle in the process. Consequently, the particle concentrations in flocculated dispersions cannot be high. The desired stabilizer should be able to produce fast milled dispersions with small particle size and low viscosity.
It is an object of the present invention to provide a wax dispersion in a solvent with a low viscosity at a shear rate less than 0.5 s−1 and where the percent of wax in the dispersion is high.
It is another object of the present invention to produce wax dispersions where the particle size is less than 1 um.
It is yet another object of the present invention to produce a wax dispersion where the time of milling is relatively short.
This and other objects of the invention can be accomplished by using a dispersant comprised of a copoloymer of styrene and butadiene, where the butadiene is partially or completely hydrogenated.